Rewinding stellar evolution: The last 400,000 years of mass loss from a star

The study of K4-37, a planetary nebula never studied in detail before, allows us to trace back the mass loss history of its last stages as a star

The study makes use of data from Calar Alto and San Pedro Martir (Mexico) observatories

Planetary Nebulae (hereafter PNe) are the last evolutionary phase of stars with initial masses between 0.8 and 8 solar masses (Msun). They appear as a compact central star (the remains of the progenitor star) surrounded by a bright shell of expanding gas, produced during the previous red giant phase, when the star blew away its external layers. In a few tens of thousand years, PNe disperse in the interstellar medium. Although more than 3,500 PNe are known to date in the Milky Way, many of them lack appropriate observations to place them in the general context of PN evolution. The study of K4-37, one of these less observed PNe, gives new hints to this context.

“Almost 20 years ago, we obtained images of K4-37 with the CAFOS instrument mounted on the 2.2-m telescope at Calar Alto Observatory. These images were taken under poor sky conditions and showed a less attractive PN. New CAFOS images obtained in 2014 under much better sky conditions revealed interesting details of the object. Thus, we planned to investigate further this PN”, points out Luis Miranda, principal investigator of the K4-37 study at the Instituto de Astrofísica de Andalucía (IAA-CSIC)

A multi-axis nebula with an old dusty shell

The new images taken at Calar Alto show that, besides its typical bipolar appearance with an equatorial torus and two bipolar lobes, K4-37 also exhibits distortions in the lobes. Moreover, the 3-dimensional reconstruction of the nebula reveals that the central torus and lobes are not perpendicular to each other, but they appear as such only due to their projection onto the sky.

“Our analysis shows the existence of three different axis in K4-37, indicating that it is not a simple bipolar, but a more complex object whose formation requires the action of several bipolar outflows along different directions. The estimated age of the nebula, about 10,000 years, implies that K4-37 is already in an advanced stage of PN evolution”, Luis Miranda (IAA-CSIC) emphasizes.

Researchers also managed to recover information on the progenitor star, which lies at around 45,700 light-years from us. By studying the abundances of some chemical elements in K4-37, which result from the nuclear reactions during the life of the star and are primarily related to its initial mass, they could infer that the progenitor star had an initial mass of 4 to 6 Msun. They could as well estimate for the optical nebula a size of about 2.3 x 0.8 light-years.

The analysis of infrared images of K4-37 available the WISE satellite archive revealed another surprising finding. “We found that the optical nebula is surrounded by a huge, dusty elliptical envelope, with a size of about 43 x 26 light-years and an age of about 430,000 years. If the age of the optical nebula approximately marks the time span since the central star entered the PN phase, the elliptical envelope should be ascribed to the previous red giant phase, during which strong mass loss occurs”, Luis Miranda (IAA-CSIC) points out.

These kind of dusty shells is observed in other evolved stars, as a result of the interaction between the gas blown away during the last evolutionary phases and the interstellar medium. Yet, it is the first time that such a structure has been detected in a very evolved PN like K4-37. In addition to its very large size, the elliptical shape of the envelope contradicts the spherical mass ejection that is expected in the red giant phase. The researchers suggest that the elliptical shape could be due to interaction with the interstellar magnetic field, or to the existence of a companion to the central star.

“There is a strong contrast between the simple shape of the elliptical envelope and the complex structure of the optical nebula. This suggests that fundamental changes in some properties of the central star have occurred just before or during the PN phase, which have created the different morphologies. Intriguingly enough, the central star of K4-37 still remains undetected although, given the complexity of this PN, a binary nature could be a good suggestion”, Luis Miranda concludes.

This research results from a collaboration between the Instituto de Astrofísica de Andalucía (IAA-CSIC, Spain), the Instituto de Astronomía-UNAM and the Sonora University (Mexico).

The German-Spanish Calar Alto Observatory is located at Sierra de los Filabres, north of Almería (Andalucía, Spain). It is jointly operated by the Instituto Max Planck de Astronomía in Heidelberg, Germany, and the Instituto de Astrofísica de Andalucía (CSIC) in Granada, Spain. Calar Alto has three telescopes with apertures of 1.23m, 2.2m and 3.5m. A 1.5m aperture telescope, also located at the mountain, is operated under control of the Observatorio de Madrid.